Thanks to everyone who helped out, what a great wealth of information:
If you have a Hotmail or other e-mail that can be accessed via a browser, e-mail your passport number, traveller's cheque numbers and any other important info to yourself. That way if you need it and are separated from your stuff, you just have to find an internet connection to retreive it.
Make sure you have a copy of your living will, and spousal Power of Attorney. You don't know what may happen, and this will give your partner some rights. Even though they may not be recognized by they country you are in, they show intent, which is almost as powerful.
Most airlines limit is 50 lbs but check yours. Don't want to get hit with extra fees. I pack my reg and computer in my carry on.
Make sure the bag is under 62 inches Lenght+Width+Depth.
Make sure you remove all the lead from your BC (Including the trim weight, oops)
Mark your equipment. You will be amazed how many people may have the same equipment that tends to all get piled up in a corner on the boat.
Batteries, sunglasses, sun lotion, meds, sandles, small dry box....
Tips are 5-10 a tank depending on the service.
Safety whistle, mirror, safety sausage, and cutting implement like surgical shears. Mask defog gel. Don't know about Cancun, but in Coz generally NO knives, and NO gloves. (Yes Cancun has the same restriction)
stack of $1 bills, passport, photocopy of passport hidden somewhere else (in case your original gets stolen). Phone numbers, addresses, and printouts for any reservations you have.
The $1 bills are for hotel tips, taxi fares, and a bunch of other places where all you might have is a $20 and no one seems to have any change....... (this is very true, also make sure you know the exchange rate so you do not get cheated, much. Change is not exchangeable, don't bother bringing it!)
Don't take your log book, you might lose it. Instead take some blank pages and put them in the log book when you get home.
If you have an EMS store by you, they carry great dry bags that pack tiny and flat. They are useful on have on the boat to keep your tip money, hat, and other small stuff dry. Speaking of that, bring a baseball cap so you can actually see when you're out in the sun and so your face doesn't burn. I don't like to use sunblock before diving because it gets in my eyes, so the hat is a must. When you're out on the water, your lips tend to get dry (at least mine do), so bring some chapstick type stuff with spf in it. Jolly Ranchers will win you friends on the boat during the SI. I bring a small save a dive kit in a zip lock bag with o rings, a spare mask strap, a few zip ties, a couple of snorkel keeprs, and a double ender. Doesn't take up much room and seems to solve most things that come up. (some great points here!)
I was tying to get the antibiotics incase we have a problem while down there. My personal doctor was not interested in helping, so I called one of the passport asistance companies. They had a list of doctors, and some had them on staff.
When you put your Octo in the carry on, be ready for it to be opened. Put everything in there that they might question! Be prepared for the TSA agent at the Xray machine to not know what it is. I was checked every time!.
Bring an extra computer battery, and the tools to change it. Save yourself a big problem later.
Don't bother with the TSA locks, they cut them off anyway. (I am now out 4 locks. They made it to Mexico, but not back, all were lost in Atlanta.) Use zipties, and put extras with a note to pls re-ziptie.
If some of your luggage is missing at first, wait around. They are very careful about checking the ownership of lugage before you leave the Cancun airport. My bag was missing for 15 min, then suddenly re-appeared. Nothing missing.
Lastly, don't try to pet the dogs in the airport
